EVALUATION OF NEW SILVER-FREE BRAZING FILLER METALS PART 2: BRAZING OF COPPER AND BRASS

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1 EVALUATION OF NEW SILVER-FREE BRAZING FILLER METALS PART 2: BRAZING OF COPPER AND BRASS The results of wetting and shear strength testing, and metallurgical characterization, of Cu-4Sn- 6P and Cu-40Zn-1Sn-0.3Si silver-free filler metals designated for brazing copper and brass are reported By Jacob T. Marchal 1, Matthew J. Duffey 1, Matthew R. Loney 1, Boian T. Alexandrov 1, and Alexander E. Shapiro 2 1 Welding Engineering Program, The Ohio State University, Columbus, OH marchal.16@buckey .osu.edu 2 Titanium Brazing, Inc., Columbus, OH ashapiro@titanium-brazing.com 1

2 Executive Summary Two silver-free filler metals: TiBraze P14 (Cu-6P-4Sn wt.%) and TiBraze LOK59-03 (Cu-40Zn-1Sn-0.3Si wt.%) were experimentally evaluated to determine if these alloys are suitable to replace silver-based filler metals for brazing copper and brass. Characteristics of the filler metals, such as spreading area, joint strength, and microstructure were analyzed. These characteristics were compared to filler metals that are currently used in the industry, such as standard silver-based BAg-1, BCuP-5, and a silver-free BCuP-9. Metallurgically compatible silver-free braze alloys used to join copper and brass base materials were found to yield ultimate joint strengths rivaling those of silver-based filler materials. Data collected based upon computational models (ThermoCalc TM ), optical microscopy, and shear tests provided substantiation of compatibility. Shear strength of brazed joints made with TiBraze P14 is higher by 21-24% than that of low-silver standard BCuP-5 filler metal. Although strengths in silver-free filler materials shear stresses are between 72% and 85% those of high silver-based alloy BAg-1, costs of joints can be reduced significantly with silver-free filler material. By increasing the overlap and using silver-free fillers, joint cost can be reduced while meeting strength requirements. Materials and procedure The base materials used in this study were copper C110 and brass C260 alloy containing 70 wt.% of copper and 30 wt.% of zinc. These materials are machinable and widely used for manufacturing hydraulic and pneumatic pipes, cartridges, ammunition casing, radiators, hardware, etc. They do not require heat treatment after welding, brazing or soldering. Two new silver-free filler metals: TiBraze P14 in the form of a brazing paste using a rubberbased binder and TiBraze LOK59-03 rods Ø2.4 mm were evaluated to determine characteristics such as spreading area, joint strength, and microstructure. Table 1 contains chemical compositions of each filler metal. Testing results of the same silver-free filler metals in combination with low-carbon steel and stainless steel are reported elsewhere (Ref. 1). Brazing Filler Metal Table 1 Brazing filler metals, their compositions and melting ranges Chemical Composition, wt% Melting range, C F TiBraze LOK59-03 Cu-39.7Zn-( )Sn-( )Si TiBraze P14 Cu-(5.3-6)P-(3.5-4)Sn BCuP-9 Cu-(6-7)Sn-(6-7)P-( )Si BCuP-5 Cu-( )Ag-( )P BAg-1 Ag-(14-16)Cu-(14-18)Zn-24Cd Two standard silver-containing filler metals BAg-1, BCuP-5 and a silver-free BCuP-9 alloy were tested for comparison with new silver-free filler metals. 2

Brazing was performed in air by heating with a propane torch. The flux used during brazing was the boron-modified, black fluoroborate flux 601B/3411 (Superior Flux & Mfg. Co., Solon, OH).

3 Brazing was performed in air by heating with a propane torch. The flux used during brazing was the boron-modified, black fluoroborate flux 601B/3411 (Superior Flux & Mfg. Co., Solon, OH). Methods, materials, and sample designs applied for testing wetting and spreading, shear strength, and microstructure characterization are described elsewhere (Ref.1). Results and Discussion Wetting Characteristics The spreading area provides an indication of how the filler metal will spread and flow into the joint during brazing. Figure 1 shows the resulting average spreading area of each filler metal on different base metals. Fig. 1 Average spreading areas of tested filler metals on Copper and Brass The spreading area provides an indication of how the filler metal will spread and perform during brazing. The filler metals with low spreading area have difficulty spreading through the 3

4 joint and creating fillets, thus decreasing the overall strength of the joint. Silver-free LOK59-03 exhibited better spreading on copper than a standard silver-based alloy BAg-1 and silver-free filler metal P14 has almost the same wetting characteristics on brass as BAg-1. At the same time P14 exhibited the worst spreading on copper among all tested brazing alloys. Shear Strength The results of the average shear strength testing for all filler metals are in Table 2 and Fig. 2. Fig. 2 Average shear strength of tested filler metals on copper and brass 4

5 Table 2 - Average shear strengths of brazed joints, for each filler metal, ksi (MPa) Braze Filler Metal Copper Brass TiBraze LOK (124.2) - TiBraze P (153.2) 29.1 (200.8) BAg (172.5) 34.5 (238.1) BCuP (122.8) 26.0 (179.4) BCuP (146.3) 22.2 (153.2) Brazing with P14 resulted in sufficient shear strength with copper and, especially, with brass - the second highest value after the standard silver-based alloy BAg-1 that makes the silver-free P14 alloy a very prospective substitute for silver. At least, 15%-silver containing BCuP-5 can be completely substituted by P14 in such applications as the manufacture of copper tubing and brass faucet brazed joints. As expected, LOK59-03 exhibited modest values of shear strength of copper brazed joints that can be explained by higher yield strength of the cast joint metal than that of the base metal, which results in limited deformability of the joint metal. The load-displacement diagram (Fig. 3a) shows low ductility of LOK59-03 brazed joint, while copper brazed joint made with P14 has mechanical behavior with some ductility of the joint metal (Fig. 3b). Brass brazed joints made with silver-free P14 alloy exhibited certain ductility and a yield point about 15.5 ksi (107 MPa). The load-displacement diagram (Fig. 3c) shows formation of several micro-cracks that, however, do not propagate through the joint metal but only release stresses, while the joint continues to resist higher shear loading. Shear strength of copper or brass brazed joints made with the silver-free P14 filler metal is greater than that of both phosphoruscontaining standard braze alloys BCuP-5 and BCuP-9. The strength of P14 joints is second only to silver-based BAg-1 brazing filler metal. (a) (b) (c) Fig. 3 Load-displacement relationships of brazed joints: (a) Low-ductile behavior of copper joints brazed with LOK59-03; (b) Moderate ductile behavior of copper joints brazed with P14; (c) Ductile behavior of brass joints brazed with P14 despite of several micro-cracks, which appeared in the joint metal 5

Macro- and Microstructures of Brazed Joints Brazed joints were examined via optical and scanning electron microscopy to characterize phase composition and solidification microstructure of each filler

6 Macro- and Microstructures of Brazed Joints Brazed joints were examined via optical and scanning electron microscopy to characterize phase composition and solidification microstructure of each filler metal. Figures 4a, b are images of a copper joint made with the P14 filler metal. Quality of this joint is characterized by dense joint metal and smooth, defect-free fillets. Phase composition of the joint metal and the amount of each phase as predicted by Thermocalc TM are represented elsewhere (Ref.1). Based on the calculations from Thermocalc TM, three phases are present in the P14 joint metal: eutectic composed of Cu 3 P and Cu 3 Sn, and an FCC phase, which is copper-tin alloy with insignificant amount of phosphorus. Eutectic phase dominates in the joint metal that provides moderate ductility and high shear strength of brazed joints 22.2 ksi (153.2 MPa). No intermetallic layers were found at the interface with the base metal. The Cu-Sn alloy phase is solidifying first along the base metal interface, while relatively low-melting eutectic is ousted into the middle of the joint, where it is crystallized at lower temperature. Apparently, the liquid reached a low enough temperature to begin to form the eutectic phase prior to the complete transformation of the FCC phase. Formation of the FCC phase predominantly occurs at the interface in the uniform eutectic microstructure of the joint metal. Fig. 4a Macrostructure of copper + P14 brazed joint, x12.5 6

7 Copper FCC phase Eutectic phase (Cu 3 P and Cu 3 Sn) Fig. 4b Microstructure of copper + P14 brazed joint, x200 Fig. 5a Macrostructure of brass + P14 brazed joint, x12.5 7

Fig. 5b Microstructure of brass + P14 brazed joint, x500 Figures 5a and 5b are images of a joint made with brass (base metal) and P14 powder (filler metal).

8 Fig. 5b Microstructure of brass + P14 brazed joint, x500 Figures 5a and 5b are images of a joint made with brass (base metal) and P14 powder (filler metal). The phase composition of the joint metal is presented by a FCC phase of a ternary solid solution Cu-(8.5-11)Zn-2Sn wt.% and an eutectic containing Cu 3 P and Cu 3 Sn components. The microstructure displays that the FCC phase grows along the joint interface with the base metal, similar to the copper base metal joint. The eutectic phase dominates the middle of the joint. Figure 5a portrays a smooth fillet and a uniform microstructure of joint metal. Some fillets contain few defects, particularly pores. It is likely that the pores appeared due to insufficient heating upon formation of the joint. Considering the relatively high strength of the brazed joints, as well as joint and fillet formation, this base metal and filler metal combination is good for brazing brass parts. 8

Fig. 6 Microstructure of copper + LOK59-03 brazed joint, x1000 Table 3 - ThermoCalc TM predicted phases and constituents in the LOK59-03 joint metal Filler Metal LOK59-03 Percentage Composition

9 Fig. 6 Microstructure of copper + LOK59-03 brazed joint, x1000 Table 3 - ThermoCalc TM predicted phases and constituents in the LOK59-03 joint metal Filler Metal LOK59-03 Percentage Composition (weight %) Phase of each phase at Cu Zn Sn Si 25 C BCC Cu 3 Sn FCC Figures 6 is an image of a joint made with copper (base metal) and LOK59-03 (filler metal). According to thermodynamic modeling with ThermoCalc TM, three phases should present the LOK59-03 joint metal (Table 3): FCC phase, BCC phase, and Cu 3 Sn intermetallic compound. The BCC phase is the high temperature phase that solidifies around 890 C. This means that the BCC phase will begin to be formed first. The entire liquid will transform into the BCC phase around 884 C, which is the solidus temperature. As the joint metal cools, the BCC phase begins to transform into the FCC phase at 689 C. Upon cooling to room temperature, most (93%) of the microstructure will have transformed into the FCC phase. There should be very little Cu 3 Sn phase, because its transformation begins at a lower temperature: ~450 C. There is insufficient time for a significant amount of Cu 3 Sn to form upon cooling from 450 C to room temperature. 9

10 However, after studying microstructure of the joint metal (Fig. 6), we suggest that the FCC phase (which is simply yellow brass) is crystallized first on the solid copper and the FCC grains grow rapidly to the middle of the joint because solidus of this brass phase concurs with the low limit of the brazing temperature range. Low-temperature phases, such as Cu 3 Sn, ousted to the central zone by said growing FCC grains. Our point of view is supported by multiple epitaxial crystal growth of the FCC phase on copper grains that is clearly seen in Fig. 6, because copper (base metal) and yellow brass component of the joint metal have the same FCC lattice. Rapid solidification of the FCC phase is evidenced by appearance of shrinkage pores in the central zone of the joint. Such pores are typical for rapidly-solidified cast structures. Same pores were also found between dendrites in the fillet area. Supposedly, these pores located along the central line cause a relatively low strength of copper joints brazed with LOK59 filler metal. Hence LOK59-03 is rather recommended for brazing steels or copper to steel, while P14 alloy more suitable for brazing copper and brass structures. Silver-free brazing filler metals P14 and LOK59-03 are suitable to substitute silver-based alloys in joining hydraulic, pneumatic, and lubrication pipelines and can be widely used in the manufacture of automobiles, refrigerators, house water pipes, machinery equipment, electronics, etc. Typical examples of brazed joints of copper, steel, and brass tubes made with these silver-free filler metals are presented in Fig. 7 and 8. Fig. 7 Copper-to-copper and copper-to-steel tubing brazed joints made with silver-free P14 and LOK59-03 filler metals 10

Fig. 8 Brass tube brazed joints made with silver-free P14 filler metal Conclusions 1. Silver-free brazing filler metals are capable of meeting strength requirements in copper and brass brazed joints.

11 Fig. 8 Brass tube brazed joints made with silver-free P14 filler metal Conclusions 1. Silver-free brazing filler metals are capable of meeting strength requirements in copper and brass brazed joints. Brazing with the low-temperature silver-free filler metal TiBraze P14 (Cu- 4Sn-6P) resulted in sufficient shear strengths with copper and, especially, with brass - the second highest value after the standard silver-based alloy BAg-1; that makes the silver-free P14 alloy a very prospective substitute for silver. At least, 15%-silver containing BCuP-5 can be completely substituted by P14 in such applications as the manufacture of copper tubing and brass faucet brazed joints. 2. When used for joining copper or brass tubes with connectors, adapters, and faucets, both tested silver-free filler metals provide formation of quality, dense brazed joints with smooth fillets. Significant cost savings can be realized with silver-free brazing filler metals, warranted even in the case that sometimes joint overlap lengthening is required. 11

12 Acknowledgment This work has been performed as a Capstone Project at the Welding Engineering Program of the Ohio State University supported by Titanium Brazing, Inc. References: 1. Duffy M. J., Marchal J. T., Loney M. R., Alexandrov B. T., and Shapiro A. E Evaluation of New Silver-Free Brazing Filler Metals, Part 1: Brazing of steels, Welding Journal, vol. 94, March. 12

HEAT-RESISTANT BRAZING FILLER METALS FOR JOINING TITANIUM ALUMINIDE AND TITANIUM ALLOYS

HEAT-RESISTANT BRAZING FILLER METALS FOR JOINING TITANIUM ALUMINIDE AND TITANIUM ALLOYS Alexander E. Shapiro* and Eugene Y. Ivanov** *Titanium Brazing, Inc., Columbus, OH, ashapiro@titanium-brazing.com